Icing after eccentric contraction-induced muscle damage perturbs the disappearance of necrotic muscle fibers and phenotypic dynamics of macrophages in mice.

Icing is still one of the most common treatments to acute skeletal muscle damage in sports medicine. However, previous studies using rodents reported the detrimental effect of icing on muscle regeneration following injury. This study aimed to elucidate the critical factors governing the impairment of muscle regeneration by icing with a murine model of eccentric contraction-induced muscle damage by electrical stimulation. Because of icing after muscle injury, the infiltration of polynuclear and mononuclear cells into necrotic muscle fibers was retarded and attenuated, leading to the persistent presence of necrotic cellular debris. These phenomena coincided with the delayed emergence and sustained accumulation of Pax7+ myogenic cells within the regenerating area. Additionally, due to icing, delayed and/or sustained infiltration of M1 macrophages was noted in accordance with the perturbed expression patterns of inflammation-related factors, including tumor necrosis factor-α (TNF-α) and interleukin-10 (IL-10). The key myogenic regulatory factors (i.e., MyoD and myogenin) involved in the activation/proliferation and differentiation of myogenic precursor cells were not altered by icing during the regenerative process. A detailed analysis of regenerating myofibers by size distribution at day 14 after muscle damage showed that the ratio of small regenerating fibers to total regenerating fibers was higher in icing-treated animals than in untreated animals. These findings suggest that icing following muscle damage blunts the efficiency of muscle regeneration by perturbing the removal of necrotic myofibers and phenotypic dynamics of macrophages rather than affecting myogenic factors.